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Abstract:

A method of characterizing formation fluid present in a subsurface earth
formation during drilling using methods for correcting the measured
concentrations of gas components in drilling mud. Gas trap values for the
gas components of interest, light hydrocarbons, are measured during mud
logging and are corrected using relative response factors, determined
from laboratory fluid analysis values and relative extraction efficiency
values. The relative response factors for each gas component of interest
can be used for correcting additional gas trap values measured in the
same well or for correcting gas trap values measured in surrounding wells
utilizing a similar drilling fluid. The corrected gas trap values for
each of the gas components of interest can be utilized to calculate
gas/oil ratios for characterizing the formation fluid from the volume of
drilling mud.

Claims:

1. A method for characterizing formation fluid present in a subsurface
earth formation, the method comprising:extracting a plurality of gas
components from a volume of drilling mud comprising formation fluid and
gases, while drilling;measuring a gas trap value for each gas component
of interest;determining a gas trap response factor for each gas component
of interest by dividing the gas trap value by a laboratory fluid analysis
value for each gas component of interest;determining a relative response
factor for each gas component of interest by dividing the gas trap
response factor for each gas component of interest by the gas trap
response factor for the gas component of interest with the lowest
molecular weight; andcalculating the corrected gas trap value for each
gas component of interest by dividing the gas trap value by the relative
response factor for each gas component of interest;wherein the corrected
gas trap value for each gas component of interest can be utilized to
calculate gas/oil ratios for characterizing the formation fluid from the
volume of drilling mud.

2. A method in accordance with claim 1, wherein the plurality of gas
components are extracted from the volume of drilling mud using a gas
trap.

4. A method in accordance with claim 1, wherein the gas components of
interest are chosen from the group of methane, ethane, propane, butane,
and/or pentane.

5. A method in accordance with claim 1, wherein the gas trap values are
measured using gas chromatography or gas chromatography-mass
spectrometry.

6. A method in accordance with claim 1, wherein the laboratory fluid
analysis value is measured using gas chromatography or gas
chromatography-mass spectrometry.

7. A method in accordance with claim 1, wherein the corrected gas trap
values are used to calculate gas to oil ratios to characterize the
formation fluid.

8. A method in accordance with claim 1, wherein the determined relative
response factors are utilized to correct gas trap values measured in the
same well at various depths.

9. A method in accordance with claim 1, wherein the determined relative
response factors are utilized to correct gas trap values measured in
surrounding wells utilizing a similar drilling fluid.

10. A method for using previously determined relative response factors for
correcting gas trap values for gas components in a drilling mud,
comprising:extracting a plurality of gas components from a volume of
drilling mud comprising formation fluid and gases, while
drilling;measuring a gas trap value for each gas components of interest;
andcalculating the corrected gas trap value for each of the gas
components of interest by dividing each gas trap value by a previously
determined relative response factor for each of the gas components of
interest;wherein, the corrected gas trap value for each of the gas
components of interest can be utilized to calculate gas/oil ratios for
characterizing formation fluids from the volume of drilling mud.

11. The method in accordance with claim 10, wherein the previously
determined relative response factor for each gas component of interest is
utilized to correct gas trap values for each of the gas components of
interest measured in surrounding wells utilizing a similar drilling
fluid.

12. A system for automatically correcting a plurality of gas trap values,
comprising:a data storage device having computer readable data including
mud logging data relating to the plurality gas trap values;a processor,
configured and arranged to execute machine executable instructions stored
in a processor accessible memory for performing a method
comprising:acquiring a gas trap value for each gas component of
interest;determining a gas trap response factor for each gas component of
interest by dividing the gas trap value by a laboratory fluid analysis
value for each gas component of interest;determining a relative response
factor for each gas component of interest by dividing the gas trap
response factor for each gas component of interest by the gas trap
response factor for the gas component of interest with the lowest
molecular weight;correcting the gas trap value for each gas component of
interest by dividing the gas trap value by the relative response factor
for each gas component of interest; andutilizing the corrected gas trap
value for each gas component of interest to calculate gas/oil ratios for
characterizing formation fluids.

13. A system as in claim 12, further comprising a user interface
configured and arranged to allow a user to adjust parameters used in
correcting the gas trap value for each gas component of interest.

14. A system as in claim 12, further comprising a user interface
configured and arranged to allow a user to adjust parameters used in the
calculation gas/oil ratios.

15. A systerm as in claim 12, further comprising a display, configured and
arranged to display a layer structure of a subsurface region from which
the formation fluid and gases were taken, based, at least in part, on the
gas trap values.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates generally to characterizing formation
fluid present in a subsurface earth formation during drilling and more
particularly to methods for correcting the measured concentrations of gas
components in drilling mud.

BACKGROUND OF THE INVENTION

[0002]In oil or gas well drilling operations, drilling fluid (or "mud") is
continuously circulated through the inside of the drill pipe and out the
drill bit then back up to the surface. Drilling mud is typically made up
of clays, chemical additives and an oil or water base. This fluid has
several purposes: 1) controlling formation pressure; 2) cleaning the well
bore of formation debris; 3) lubricating, cooling, and cleaning the drill
bit and drill string; 4) stabilizing the well bore; and 5) limiting the
loss of drilling mud to the subsurface formation.

[0003]In cleaning the well bore, the circulating drilling mud removes-the
drill cuttings as well as formation fluid trapped in the pore space or
fractures of the rock. During the drilling operation, the entrapped
formation fluid and gases in the drilling mud are monitored in real-time
at the surface. The recording of the measurements is called mud logging.
Mud logging measurements can include temperature, pH, drill rate,
chlorides, total hydrocarbon content, and the concentration of specific
formation gas components. These logs are important as they enable the
drilling operator to ascertain the presence of oil or gas in the
formation being drilled. Significant measured gas increases in the
drilling mud during drilling indicate oil or gas bearing zones in the
formation and are known as "shows".

[0004]To measure the amount of formation gas entrapped in the drilling mud
and determine the concentration in the formation fluid, several
techniques have been used. A small amount of the drilling mud can be
pumped through a mechanical agitation device known as a gas trap which is
located at the surface. The purpose of the gas trap is to extract the
gases from the drilling mud for measurement and analysis. Separation and
quantification of the gas components, light hydrocarbon gases, is
typically carried out by means of in-line gas-chromatography or
gas-chromatography mass-spectrometry analysis. Gas trap sampling and
analysis can be monitored continuously in real-time as part of typical
mud logging activities, providing the drilling operator with real-time
concentrations of the gas components per linear foot drilled for the
entire depth of the well. The ability to distinguish formation fluid
types, especially their gas to oil ratios (GOR), from analysis of the
light gases is a highly desirable goal since time and resources spent on
formation testing can be minimized. This data is essential for the
economics and feasibility of any hydrocarbon reservoir.

[0005]Unfortunately, there are numerous problems associated with surface
gas trap measurements. The relative concentrations of the different gas
components extracted from the drilling mud and collected in the head
space of the gas trap are not representative of the actual gas
concentrations evolving from the drilling fluid. As a result, measured
gas trap values are not representative of the gas composition of the
drilling fluid or the formation fluid at depth. Additionally, uncorrected
gas trap values can lead to widely divergent predictions of total fluid
properties, including GOR determinations.

SUMMARY OF THE INVENTION

[0006]Aspects of embodiments of the present invention provide a method for
characterizing formation fluid present in a subsurface earth formation,
including, extracting a plurality of gas components from a volume of
drilling mud containing formation fluid and gases, while drilling,
measuring a gas trap value for each gas component of interest,
determining a gas trap response factor for each gas component of interest
by dividing the gas trap value by a laboratory fluid analysis value for
each gas component of interest, determining a relative response factor
for each gas component of interest by dividing the gas trap response
factor for each gas component of interest by the gas trap response factor
for the gas component of interest with the lowest molecular weight, and
calculating the corrected gas trap value for each gas component of
interest by dividing the gas trap value by the relative response factor
for each gas component of interest, for characterizing the formation
fluid from the volume of drilling mud.

[0007]In an embodiment, the method further includes a method for using
previously determined relative response factors for correcting gas trap
values for gas components in a drilling mud, including, correcting gas
trap values for each of the gas components of interest measured in
surrounding wells utilizing a similar drilling fluid.

[0008]Aspects of embodiments of the invention provide a system for
performing the foregoing method. Aspects of embodiments of the invention
may include a computer-readable medium encoded with computer-executable
instructions for performing the foregoing method or for controlling the
foregoing system. Aspects of embodiments of the invention may include a
system incorporating the foregoing system and configured and arranged to
provide control of the system in accordance with the foregoing method.
Such a system may incorporate, for example, a computer programmed to
allow a user to control the device in accordance with the method, or
other methods.

[0009]These and other objects, features, and characteristics of the
present invention, as well as the methods of operation and functions of
the related elements of structure and the combination of parts and
economies of manufacture, will become more apparent upon consideration of
the following description and the appended claims with reference to the
accompanying drawings, all of which form a part of this specification,
wherein like reference numerals designate corresponding parts in the
various Figures. It is to be expressly understood, however, that the
drawings are for the purpose of illustration and description only and are
not intended as a definition of the limits of the invention. As used in
the specification and in the claims, the singular form of "a", "an", and
"the" include plural referents unless the context clearly dictates
otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a flow chart illustrating a method in accordance with one
or more embodiments of the invention.

[0011]FIG. 2 is an example mudlog showing gas trap values for the
formation gas components measured during drilling in accordance with one
or more embodiments of the invention.

[0012]FIG. 3 is a flow chart illustrating a method in accordance with one
or more embodiments of the invention.

[0013]FIG. 4 is an, example mudlog showing gas trap values for the
formation gas components and calculated GOR values in accordance with one
or more embodiments of the invention.

[0014]FIG. 5 is a schematic illustration of an embodiment of a system for
performing methods in accordance with one or more embodiments of the
invention.

DETAILED DESCRIPTION

[0015]Referring to FIG. 1, a method 10 for characterizing formation fluid
present in a subsurface earth formation is shown. During drilling
operations, a plurality of formation gas components are extracted 12 from
a volume of circulating drilling mud which contains formation fluid and
gases. In one embodiment, the formation gas components are extracted from
the mud by mechanical agitation in a gas trap located at the surface. Gas
trap values 14 for each gas component of interest is measured. Separation
and quantification of the different formation gas components of interest,
typically any of the light hydrocarbon gases (methane through pentane),
is measured by means of an in-line gas-chromatograph or gas-chromatograph
mass-spectrometer, and/or a total hydrocarbon analyzer. While the means
for such an analysis is preferably a gas chromatograph equipped with a
flame ionization detector for hydrocarbon gases, it will be appreciated
that any means for analyzing the gas mixture composition could be
utilized.

[0016]Table 1 shows correct gas trap values in accordance with an
embodiment of the present invention. The concentration of each gas
component in the drilling mud can be determined in parts per million
(ppm) and percent of the total formation gases as shown in rows 1 and 2
in Table 1. FIG. 2 shows an example mudlog 22 including real-time
continuous gas trap values 24 for the formation gas components measured
during drilling.

[0017]Periodic laboratory fluid analysis measurements taken directly from
the formation fluid are made in order to characterize the formation
fluid. This data can then be used to calibrate or correct the gas trap
values. In one embodiment, the correction can consist of collecting a
known volume of drilling mud at the gas trap and then distilling the
sample in a steam or microwave still. The distilled gas is then measured
for each gas component. In another embodiment, the calibration can
consist of collecting a sample of formation fluid downhole utilizing an
apparatus similar to Schlumberger Limited's Modular Formation Dynamics
Tester (MDT) tool or Repeat Formation Tester (RFT) tool. Standard
pressure-volume-temperature (PVT) laboratory fluid analysis can then be
carried out on the formation fluid to determine the concentration of each
gas component of interest in the formation fluid. While the means for
such an analysis is preferably a gas chromatograph equipped with a flame
ionization detector for hydrocarbon gases, it will be appreciated that
any means for analyzing the gas mixture compositions could be utilized.

[0018]The laboratory fluid analysis values indicating the concentration of
each gas component in the formation fluid are determined in mole % and
percent of the total formation gases as shown in rows 3 and 4 in Table 1.
In an embodiment, the laboratory fluid analysis values in Table 1 were
established using formation fluid samples collected from a MDT run on the
same well and at the same approximate depth, used to determine the gas
trap values. Referring back to FIG. 1, the laboratory fluid analysis
values are used to determine a gas trap response factor 16 for each gas
component of interest as shown in row 5 of Table 1. The gas trap response
factor can be determined by dividing the gas trap value in row 2 by the
laboratory fluid analysis value in row 4 of Table 1, for each gas
component of interest.

[0019]The relative concentrations of the different gas components
extracted from the drilling fluid and collected in the head space of the
gas trap are not representative of the actual gas concentrations evolving
from the drilling fluid. This is largely due to the extraction efficiency
of the different gas components. Light, hydrocarbons are extracted as a
function of their carbon number (volaility and solubility) i.e. methane,
is extracted easier than ethane, and ethane is extracted easier than
propane. In order to accurately determine the concentration of each gas
component, the extraction efficiency relative to methane, or the lightest
measured gas component of interest, is also needed to correct the
measured gas trap response factor for each gas component.

[0020]A relative response factor is determined 18 for the gas components
of interest by dividing the gas trap response factor for each gas
component of interest by the gas trap response factor for the gas
component with the lowest molecular weight. In the example provided in
Table 1, the relative response factors in row 6 are calculated using the
gas trap response factor of methane, typically the lowest molecular
weight gas component; however, it will be appreciated that ethane could
be utilized in the absence of methane and propane could be Utilized in
the absence of methane and ethane.

[0021]The corrected gas trap value for each of the gas components of
interest is determined 20 by dividing the gas trap value in row 1 by the
relative response factor in row 6 for each :gas component of interest.
The corrected gas trap values in row 7 and 8, are more representative of
the formation fluid than the original gas trap values in rows 1 and 2 as
they match the laboratory fluid analysis values in row 4. Gas trap values
corrected only with laboratory fluid analysis values without correction
for extraction efficiency are not as representative of the gas
composition in the formation.

[0022]In another embodiment, the determined relative response factors
shown in row 6 of Table 1 may be applied to the gas trap data for the
rest of the well as shown in Table 2, to correct all the measured the gas
trap values at all depths.

[0023]Referring to FIG. 3, a method 30 for correcting gas trap values for
gas components of interest in a drilling mud using previously determined
relative response factors is shown. As in the previous example, a
plurality of gas components are extracted 32 from a volume of circulating
drilling mud which contains formation fluid and gases. The gas trap
values are measured 34 for the gas components of interest. The corrected
gas trap value for each of the gas components of interest is calculated
36 by dividing each gas trap value by a previously determined relative
response factor for each gas component. The corrected gas trap values in
row 5 of Table 2, is more representative of the laboratory fluid analysis
values in row 4 of Table 1, than the measured uncorrected gas trap values
in rows 1 and 2 of Table 2.

[0024]In an embodiment, the method utilizes available laboratory fluid
analysis values to determine relative response factors for correcting gas
trap values, collected during drilling, to better characterize formation
fluids in zones or at depths, where laboratory fluid analysis may not be
available. It will be appreciated that previously determined relative
response factors can also be utilized to correct the gas trap values
measured in surrounding wells using a similar drilling fluid. Gas
extracts from a water-based mud easier than an oil-based mud, therefore,
the relative response factors of gas components in water-based mud are
significantly higher than in oil-based mud. The method is helpful for
characterizing the relative response of the gas trap in more complex
drilling fluid systems, such as oil-based mud systems.

[0025]The corrected gas trap values can be taken as representative of the
gas composition of the formation fluid and used for predicting gas/oil
ratios (GOR). The ability to distinguish formation fluid types,
especially their GOR, from analysis of the formation gas components is a
highly desirable goal since time and resources spent on formation testing
can be minimized. As an example, GOR can be calculated as shown equation
(1):

GOR (SCF/bbl)=100,00019 [C1+C2+C3+C4]/(Rel Weight Oil) (1)

[0026]Where C1, C2, C3 denote methane, ethane and propane in molar
concentration (% or ppm by mole); and C4 and C5 denote butane and pentane
with all isomers being totaled in molar concentration (% or ppm by mole).

[0027]The relative weight of oil (Rel Weight Oil) can be calculated for
methane through pentane, as shown in equation (2):

Rel Weight Oil=3070(C3C52)/C4sqrt(C2C4) (2)

or if pentane is not monitored, as shown in equation (3):

Rel Weight Oil=1932C42/sqrt(C2C3) (3)

[0028]FIG. 4 generally shows a method of predicting GOR using relative
response factors from an adjacent well to correct gas trap values, in
accordance with an embodiment of the invention. Both wells have an oil
based mud system. The mud log data 40 shows the real-time monitoring of
drill rate (ROP), gamma ray (GRNORM) data, depth, corrected gas trap
values for methane through pentane 42, and calculated GOR. The calculated
GOR values 44 were calculated using previously determined relative
response factors from an adjacent well as shown in Table 3.

[0031]The calculated GOR 44 using the corrected gas trap values closely
matched the reported GOR of 1136 SCF/bbl calculated directly from the
laboratory fluid analysis values, as shown at depth 46 in FIG. 4.
Likewise, the calculated GOR using the corrected gas trap values was 720
scfs/bbl which closely matched the reported GOR of 750 SCF/bbl calculated
directly from the laboratory fluid analysis values, as shown at depth 48.
As illustrated, uncorrected gas trap values are not representative of the
actual gas concentrations evolving from the drilling fluid and can lead
to widely divergent predictions of formation fluid properties, including
GOR determinations. Accurate predictions of formation fluid properties
and GOR determinations can be made using relative response factors to
correct the gas trap values for gas components in a drilling mud.

[0032]A system for performing the method is schematically illustrated in
FIG. 5. A system 50 includes a data storage device or memory 52. The
stored data may be made available to a processor 54, such as a
programmable general purpose computer. The processor 54 may include
interface components such as a display 56 and a graphical user interface
58. The graphical user interface (GUI) may be used both to display data
and processed data products and to allow the user to select among options
for implementing aspects of the method. Data may be transferred to the
system 50 via a bus 60 either directly from a data acquisition device, or
from an intermediate storage or processing facility (not shown).

[0033]Although the invention has been described in detail for the purpose
of illustration based on what is currently considered to be the most
practical and preferred embodiments, it is to be understood that such
detail is solely for that purpose and that the invention is not limited
to the disclosed embodiments, but, on the contrary, is intended to cover
modifications and equivalent arrangements that are within the spirit and
scope of the appended claims. For example, though reference is made
herein to a computer, this may include a general purpose computer, a
purpose-built computer, an ASIC programmed to execute the methods, a
computer array or network, or other appropriate computing device. As a
further example, it is to be understood that the present invention
contemplates that, to the extent possible, one or more features of any
embodiment can be combined with one or more features of any other
embodiment.